The present invention is directed to the extinction of electric arcs in electric circuit breakers, particularly current-limiting electric circuit breakers.
A current-limiting circuit breaker is generally understood to be that type of high current interrupting capacity circuit breaker capable of substantially limiting the duration and the intensity of current destined to flow in a circuit experiencing a short circuit fault. To limit the duration and the intensity of short-circuit currents, a circuit breaker must, within the shortest possible time, separate its contacts and extinguish the resulting electric arcs.
To promote a better understanding of current limiting, the following definitions are set forth. "Presumed short circuit current" is that current which would flow in a circuit subjected to a short circuit fault, i.e., a fault whose impedance is essentially zero. The magnitude of the presumed short circuit current depends upon the impedance of the circuit upstream from the short circuit fault and the current available of the source feeding the fault. "Effective short circuit current" is the actual short circuit current that is let through by the circuit breaker during its interruption process. "Interruption time" of a circuit breaker is the time taken by the circuit breaker to interrupt a short-circuit current from its inception and is composed of the sum of the "intervention time" (the time required to effect breaker contact separation) and the "arc time" (the time required to fully extinguish the resulting arc). "Arc voltage" is the voltage appearing across the footpoints of the arc, which is in opposition to the source driving voltage and thus acts to diminish the magnitude of the effective or let-through short circuit current. From this it is seen that the higher the arc voltage, the lower the magnitude of the effective short circuit current the circuit breaker lets through.
Thus, a current-limiting circuit breaker must operate such as to shorten both the time of intervention and the time of extinction of the arc by increasing arc voltage in a very short time, on the order of milliseconds.
A known solution for limiting the duration and magnitude of effective short circuit current is to use current-limiting fuses designed to effect interruption of the circuit and extinction of the resulting arc within the requisite short time. While this solution is rather effective, it suffers from the grave disadvantage that the fuses must be replaced after each interruption and, in the case of a three-phase circuit, a so-called "single phasing" situation is created if only one of the three fuses blows. To remedy the latter negative aspect, it is known to integrate such fuses with a circuit breaker having a modest interrupting capacity, such that the circuit breaker is automatically tripped open to interrupt all of its three poles in response to the blowing of any one of the fuses. However, it does not avoid the need to replace blown fuses and is somewhat expensive.
Another approach to current limitation is to use high-speed actuators of the electromagnetic type, such as described in U.S. Pat. No. 1,763,502. Such actuators act directly on the breaker contacts to effect their separation whenever the line current flowing through it exceeds a predetermined value.
Still another approach resides in utilizing the electrodynamic forces associated with the currents feeding the breaker contacts being made to flow in opposite directions along closely spaced parallel paths and thereby develop repulsion forces effective in achieving rapid contact separation. This approach has been variously and differently applied at times and can be effective from the point of view of intervention time and rapidity of contact separation. However, there remains the very considerable problem of rapidly extinguishing the arc.
Swift extinction of the arc usually entails the resort to electromagnetic or pneumatic means for motivating the arc so as to increase its path length, promote removal of the arc from the breaker contacts, and facilitate cooling and splitting up of the arc; all contributing to increasing the arc voltage to a value in excess of the system driving voltage.
Among the devices for achieving ultimate quenching of the arc, the most typical is an arc chute having a given number of superimposed ferromagnetic plates separated from one another and provided with appendices or horns embracing the path of the arc drawn between the contacts. This plate configuration is effective in drawing the arc into the arc chute where it is cooled and split up into a plurality of arclets. Another type of arc chute is formed of metallic plates bent in U-shape, with the curve of the U facing the contacts, such as illustrated in U.S. Pat. No. 1,925,858. These patent plates should promote a more intense electrodynamic action on the arc due to the currents flowing in the arms of the U. However, from the patent description it does not appear that especially favorable results were obtained, and, in order to avoid plate damage, it is suggested that they be coated with a highly conductive material, such as copper.
The apparent failure of this type of U-shaped plate may be explained by the fact that the plates are necessarily of a considerable thickness, thus limiting the number of plates that can be physically accommodated in a typical arc chute. Consequently the ability of the arc chute to cool and split up the arc pursuant to effecting an ultimate quench is diminished.
It is accordingly an object of the present invention to provide an improved current-limiting electric circuit breaker of high current interrupting capacity.
Another object is to provide a current-limiting circuit breaker of the above character which is equipped with improved means for rapidly extinguishing the arcs drawn between the breaker contacts.
An additional object is to provide a current limiting circuit breaker of the above character which is further equipped with improved means for motivating the arc into the arc extinguishing means.
Yet another object is to provide a current-limiting circuit breaker which is efficient in construction, compact in size and reliable in operation.